The present application relates to a battery including a cathode, an anode, and an electrolytic solution.
In recent years, portable electronic devices such as combination cameras (videotape recorder), mobile phones, and notebook personal computers have been widely used, and it is strongly demanded to reduce their size and weight and to achieve their long life. Accordingly, as a power source for the portable electronic devices, a battery, in particular a light-weight secondary batter capable of providing a high energy density has been developed.
Specially, a secondary battery using insertion and extraction of lithium (Li) for charge and discharge reaction (so-called lithium ion secondary battery) is extremely prospective, since such a secondary battery provides a higher energy density compared to a lead battery and a nickel cadmium battery.
As a composition of material for the lithium ion secondary battery, the composition in which a carbon material is used as an anode active material of the anode, a complex oxide of lithium and a transition metal is used as a cathode active material of the cathode, and a mixture of ester carbonate is used as a solvent of the electrolytic solution is known. Since ester carbonate has superior oxidation resistance and superior reduction resistance compared to water or other organic solvent, a high voltage is thereby obtained.
Further, as a battery structure of the lithium ion secondary battery, a laminated film type structure using a package member such as an aluminum laminated film is practically used, since the laminated film type lithium ion secondary battery is lightweight and has a high energy density. In particular, a secondary battery in which an electrolytic solution is held by a polymer compound to obtain a gelatinous state (so-called polymer secondary battery) is widely used, since deformation of the package member is thereby inhibited.
To further obtain a higher capacity in the lithium ion secondary battery using the carbon material as an anode active material, as one method, there is a method to increase the occupancy ratio of the anode active material in the battery by increasing the thickness of the anode active material layer and increasing the volume density. To increase the thickness of the anode active material layer is hereinafter referred to as “thickening of the anode active material layer,” and to increase the volume density of the anode active material layer is hereinafter referred to as “increase of the volume density of the anode active material layer.”
However, when the thickness of the anode active material layer is increased and the volume density of the anode active material layer is increased, while a higher capacity is obtained, the impregnation characteristics of the electrolytic solution to the anode active material layer are lowered or intercalation efficiency of lithium ions in charge is lowered. Accordingly, in some cases, lithium becomes a dendrite to be precipitated and loses its activity. In the result, internal short circuit may be generated and thus the cycle characteristics may be lowered.
As a method to solve the foregoing issue, there is a method in which as a solvent of the electrolytic solution, a chain ester carbonate (low-viscosity solvent) such as dimethyl carbonate and ethyl methyl carbonate is used. However, when the chain ester carbonate is used in the laminated film type secondary battery, while intercalation efficiency of lithium ions is improved, the secondary battery is easily swollen due to gas generated under the high temperature atmosphere, and the cycle characteristics are lowered as a result. In this case, it is possible to inhibit the secondary battery from being swollen by using diethyl carbonate as the chain ester carbonate. However, when the thickness of the anode active material layer is increased and the volume density of the anode active material layer is increased, the cycle characteristics are drastically lowered.
Further, as another method to solve the foregoing issue, there is a method in which propylene carbonate or γ-butyrolactone having a high electric conductivity is used together with ethylene carbonate or various chain ester carbonates to improve insertion ability of lithium ions. However, when propylene carbonate or the like is used in the case of using a carbon material as an anode active material, the propylene carbonate or the like is reacted with the anode and is decomposed, and thus the cycle characteristics are lowered. Such a tendency is significant when graphite is used as an anode material.
In addition, to solve the foregoing issue, a method to add various additives to the electrolytic solution has been considered. Specifically, to improve the cycle characteristics, the storage characteristics, the load characteristics or the like, as an additive, a sulfone-based compound such as a chain disulfonic acid anhydride (for example, refer to Japanese Patent No. 3760539, Japanese Unexamined Patent Application Publication No. 2004-253296, and Japanese Unexamined Patent Application Publication No. 2006-344391), a cyclic disulfonic acid anhydride (for example, refer to Japanese Unexamined Patent Application Publication No. 2004-022336), a chain sulfonic acid/carboxylic acid anhydride (for example, refer to Japanese Unexamined Patent Application Publication No. 2002-008718), and a cyclic disulfonic acid ester compound (for example, refer to Japanese Unexamined Patent Application Publication Nos. 2005-135701 and 2005-228631) is used. In such a method, the sulfone-based compound is decomposed in the initial charge and discharge, and a coat is formed on the surface of the electrode. Thereby, intercalation efficiency of lithium ions in charge is improved and decomposition of the electrolytic solution is inhibited, and thus the cycle characteristics and the like are improved.